Electrophysiological and morphological study of polyneuronal innervation in the cutaneous pectoris muscle of adult frog (Rana esculenta)

Acetylcholine receptor bars and transmitter release in frog neuromuscular junctions

After labelling with rhodaminated alpha-bungarotoxin, acetylcholine receptors in cutaneous pectoris muscles of normal adult frogs (Rana temporaria) appear as brightly fluorescent straight bars, usually extending over the whole gutter. Here we investigated first whether receptor bars can undergo changes and secondly whether they would provide a structural correlate for the strength of a junction. Bars of low fluorescence intensity, as well as short or discontinuous receptor bars consisting of two or three segments, suggest plasticity at the receptor/active zone level. In order to elucidate this notion, receptor bars were studied at different seasons which have previously been shown to be associated with structural changes. In two groups of frogs kept under laboratory conditions simulating wintertime and summertime, respectively, the length and number of receptor bars and the amount of discontinuous bars were investigated. Synaptic contact length, which is the summed length of labelled synaptic branches, and the number and total length of receptor bars did not differ significantly. A clear difference between Group I ("winter" frogs) and Group II ("summer" frogs) was found in the number of discontinuous bars, which was almost twice as high in Group I compared with Group II (6.4 +/- 3.3% S.D. vs 3.4 +/- 1.3% S.D., n = 8 and 7 muscles, respectively, P < 0.05). In addition, the average length of individual bars was slightly longer in Group I frogs (2.16 +/- 0.7 micron S.D. vs 2.07 +/- 0.12 microns S.D., 0.1 < P < 0.05). Transmitter release has been shown to be different in these two groups--as determined from endplate potential measurements in tubocurarine-containing bathing solutions--although it was equal when measured in low Ca2+/high Mg2+ [Dorlöchter M. et al. (1991) Pflügers Arch. 418, Suppl. 1, R31]. We also investigated whether receptor bars would be a reasonable structural correlate of synaptic function by comparing different measures of transmitters release with different structural parameters in 19 identified junctions. The mean quantal content (m) of a junction was positively correlated with the number and total length of receptor bars, but not with synaptic contact area or length. Amplitudes of the first, maximum, and plateau endplate potentials (corrected for a common resting potential and apparent input resistance) at tetanic nerve stimulation (40 Hz for 2 s) in tubocurarine block were strongly correlated with both synaptic contact length and total receptor bar length (r = 0.90 for maximum endplate potential); correlations between m and any structural measure were significantly worse.(ABSTRACT TRUNCATED AT 400 WORDS)

Design and control of the head retractor muscle in a turtle, Pseudemys (Trachemys) scripta: II. Efferent innervation

The head retractor muscle (RCCQ) of Pseudemys scripta is a useful model in which to study the mechanisms animals use to vary the force and timing of movement. Single fibers in this muscle differ significantly in attachments, length, diameter, taper characteristics, and histochemical properties, suggesting that they may be energetically and architecturally specialized for different roles in head movement. In the present paper, we report the peripheral and central efferent innervation of these diverse muscle cells, and we ask how the design of the neural apparatus is matched to the properties of its target muscle fibers. Three out of four bellies in RCCQ are supplied by multiple segmental nerves. The territories of these nerves are separated rostrocaudally within the muscle belly; thus, long muscle fibers cross the territories of two or more segmental nerves. Motor terminals in RCCQ resemble those on frog twitch muscles. Their sizes (length, bouton number) are correlated with the diameters of their target muscle fibers. Each muscle fiber bears 2-14 terminals evenly spaced (approx. 5 mm apart) along its length. Thus, single muscle fibers in RCCQ are multiterminally, and long fibers are multisegmentally innervated. Control experiments indicate that the axons in each segmental nerve arise from different motor neuron populations. Thus, short, in-series fibers are supplied by different motor neurons, and individual long fibers in RCCQ are polyneuronally innervated. These data help explain how long muscle fibers with relatively slow conduction speeds can generate rapid head movements, and they raise questions about the central mechanisms that coordinate the recruitment of RCCQ motor neurons.

Redeployment of trigeminal motor axons during metamorphosis

As a consequence of the degeneration and replacement of the jaw muscle fibers in the leopard frog, Rana pipiens, trigeminal motoneurons innervate different targets before and after metamorphosis. This investigation examined the morphological correlates of the reassignment of trigeminal motoneurons during the initial phases of myofiber turnover. Specifically, silver-cholinesterase histochemistry and electron microscopy were used to 1) identify the fate of motor axons within the neuromuscular junctions (NMJs) applied to degenerating larval myofibers and 2) to determine the origin(s) of the motor axons that innervate the postmetamorphic muscle fibers of the jaw. The results demonstrate that the NMJs are retained on larval myofibers throughout their degeneration and are readily identifiable on the residual larval basal laminae that remain after involution of the sarcoplasm. Light and electron microscopic observations provide evidence that both pre- and post-synaptic elements are present on the degenerating fibers. Furthermore, morphometric analyses indicate that the preponderance (86%) of motor axons supplying adult muscle fibers originates from the larval NMJs. This condition suggests that metamorphic redeployment of trigeminal motoneurons occurs through the resumption of growth at the axon terminal supplying larval muscle rather than through the proximal collateralization of these axons and resorption of larval terminals.

Probabilistic secretion of quanta from the release sites of nerve terminals in amphibian muscle modulated by seasonal changes

The number of quanta secreted from release sites along visualized terminal branches at toad (Bufo marinus) neuromuscular junctions during different seasons was determined. Terminal branches were visualized by prior staining with the fluorescent dye, 3,3'-diethyloxadicarbocyanine iodide (DiOC2(5)). The average number of quanta secreted by release sites and recorded with an extracellular electrode was determined at different positions along terminal branches in winter and summer. There was a decrease in me at different positions along terminal branches in winter compared with me at similar positions in summer during each of 3 years. The observations show that the capacity of release sites to secrete quanta is modulated by seasonal changes.

Postmetamorphic development of neuromuscular junctions and muscle fibers in the frog cutaneous pectoris

Synaptic size, synaptic remodelling, polyneuronal innervation, and synaptic efficacy of neuromuscular junctions were studied as a function of growth in cutaneous pectoris muscles of postmetamorphic Rana pipiens. Recently metamorphosed frogs grew rapidly, and this growth was accompanied by hypertrophy of muscle fibers, myogenesis, and increases in the size and complexity of neuromuscular junctions. There were pronounced gradients in pre- and postsynaptic size across the width of the muscle, with neuromuscular junctions and muscle fibers near the medial edge being smaller than in more lateral regions. The incidence of polyneuronal innervation, measured physiologically and histologically, was also higher near the medial edge. Growth-associated declines in all measures of polyneuronal innervation indicated that synapse elimination occurs throughout life. Electrophysiology also demonstrated regional differences in synaptic efficacy and showed that doubly innervated junctions have lower synaptic efficacy than singly innervated junctions. Repeated, in vivo observations revealed extensive growth and remodelling of motor nerve terminals and confirmed that synapse elimination is a slow process. It was concluded that some processes normally associated with embryonic development persist long into adulthood in frog muscles.

Repeated, in vivo observation of frog neuromuscular junctions: remodelling involves concurrent growth and retraction

The fluorescent dye 4-(4-diethylaminostyryl)-N-methylpyridinium iodide was used as a vital stain to study remodelling of motor nerve terminals in sartorius muscles of living frogs (Rana pipiens). Identified terminals were observed twice in vivo at intervals of 87-192 days. After the second observation, muscles were fixed and stained with the nitroblue tetrazolium method for nerve terminals and with cholinesterase stain. Observations were made of 243 junctions in 26 frogs. Most nerve terminals grew during the observation interval, with an average increase in total terminal length of 29%. This growth involved substantial remodelling. Within single junctions, the change in size was the net result of differing degrees of growth or shrinkage in individual nerve terminal branches. At least one new terminal branch appeared in 25% of the junctions. Terminal retraction was also common, with branch shortening seen in 60% of junctions and the complete disappearance of a branch in 12%. In one case the original axonal input retracted completely and the junction was partially reinnervated by a terminal sprout from a junction on an adjacent fibre. Some discrepancies between histological and in vivo observations of remodelling were noted. These observations confirm that frog neuromuscular junctions are highly dynamic synapses, subject to profound structural remodelling throughout adult life.

Mechanisms of elimination, remodeling, and competition at frog neuromuscular junctions

Mechanisms governing synapse elimination, synaptic remodeling, and polyneuronal innervation were examined in anatomical and electrophysiological studies of frog neuromuscular junctions. There was a substantial level of polyneuronal innervation in adult junctions and this varied seasonally. Nerve terminal retraction and synapse elimination occurred during normal growth and following reinnervation. Synapse elimination was not inevitable, however. Repeated in vivo observations of some identified junctions showed that polyneuronal innervation could persist for over a year, while at other junctions it arose de novo by terminal sprouting. We concluded that polyneuronal innervation in adult muscles was governed by an equilibrium between processes of retraction and elimination on one hand, and sprouting and synaptogenesis on the other. Other observations revealed that structural remodeling was a common feature of adult junctions. Most often, remodeling involved the simultaneous growth and retraction of different parts of the same junction. The net result was usually junctional growth that, in small frogs, appeared to provide a good match between synaptic size and the electrical demands of transmission. In larger animals, pre- and postsynaptic sizes were not as well matched, providing morphological evidence for a growth-associated decline in synaptic efficacy. Finally, electrophysiology was used to describe some of the functional correlates and consequences of competitive interactions between the terminals of different axons. These results are explained by a hypothetical mechanism that involves trophic support provided by the muscle to the motoneuron, the overall level of nerve-muscle activity, and the synchrony of pre- and postsynaptic activity.

Muscle fibre loss and reinnervation after long-term denervation

Cutaneous pectoris muscles of frog (Rana temporaria) were investigated 19.5-40 months after denervation. On whole mounts a heavy reduction in size and number of muscle fibres is noticed; in two muscles studied with semithin and ultrathin sections the number of remaining muscle fibres is 149 and around 120, while one of the contralateral muscles contains 250 and control muscles of equal sized frogs between 220 and 320 (n = 18) fibres. By electron microscopy muscle fibres undergoing degeneration or phagocytosis can be seen (3 of 20 muscle fibres present in a single ultrathin cross-section). On the other hand several profiles contained within one common basal lamina sheath are present in 14 of 20 fibres, indicating satellite cell proliferation. In one preparation 40 months after denervation not a single muscle fibre or axon is present, suggesting that eventually, without nerve supply, muscle fibres entirely disappear. Upon spontaneous reinnervation or implantation of the hypoglossal nerve 16 months after denervation, synapses are formed with the remaining muscle fibres. When studied 3.5-24 months after nerve implantation muscles innervated by few axons only (less than 10, 10-20 axons) contain a low number of muscle fibres (mean 44 +/- 41 SD, n = 6), while all muscles with a larger number of axons have more than 150 muscle fibres (n = 6). This indicates that unless large numbers of axons regenerate and/or when reinnervation is delayed muscle fibre loss continues to occur. The presence in one muscle of motor axons but only six muscle fibres 24 months after nerve implantation indicates that muscle fibre loss cannot be reversed, or recovery is extremely slow. This observation is interpreted as evidence for the exhaustibility of the satellite cell pool.

Terminal nerve sprouting at the frog neuromuscular junction induced by prolonged tetrodotoxin blockade of nerve conduction

The effects of a prolonged blockade of nerve conduction by tetrodotoxin on frog motor innervation were studied in the cutaneous pectoris muscle of Rana esculenta. Prolonged nerve blockade (up to 22 days) was obtained by repeated subperineural injections of tetrodotoxin. Changes in morphological parameters of neuromuscular junctions were investigated in muscles after staining with a combined cholinesterase-silver method. In addition, changes in the incidence of polyneuronal innervation were investigated conjointly by electrophysiology and morphology. Morphometric analysis of singly innervated muscle fibres of 60 microns diameter revealed insignificant changes during the first week of tetrodotoxin-nerve blockade. After 15 days of paralysis, the mean length of synaptic contacts and the mean length of terminal arborization per synapse were significantly increased as compared to controls (contralateral muscles and citrate buffer-injected controls). After 20-22 days, differences in synaptic and aborization mean lengths were accentuated and reached 44 and 43%, respectively. At that time, the mean number of terminal branching points and of continuous synaptic contacts were also significantly increased (around 20 and 50%, respectively). No changes in the length of abandoned gutters were observed. The incidence of focal polyaxonal innervation (detected morphologically) and of polyneuronal innervation (determined electrophysiologically) was unchanged. The results show that prolonged tetrodotoxin blockade induces sprouting of the terminal arborization which results in an extension of pre-existing nerve terminals and an increase in the complexity of terminal arborization by addition of new branches. Nodal (collateral) sprouting was not changed.

Sprouting of frog motor nerve terminals after long-term paralysis by botulinum type A toxin

A single sublethal injection of botulinum type A toxin (BoTx-A) to winter frogs induced a general and complete paralysis of skeletal muscles, which lasted several months. Quantitative analysis of 483 end-plates from 8 BoTx-A poisoned muscles and 495 endplates from 8 control muscles revealed a higher and significant incidence of terminal and ultraterminal sprouts in poisoned junctions when taking into account the normal remodelling of motor innervation. We conclude that prolonged neuromuscular blockade by BoTx-A results in the extension of the nerve terminal arborization.